This invention relates to a method and an apparatus for judging occurrence of crushes on a body wall of a can being run through a double seamer.
Conventionally, there are known methods of judging occurrence of crushes on a body wall of a can. Japanese patent laid-open publication Nos. 211548/87, 228151/87, 239043/87 and 53450/88 have disclosed optical methods, and Japanese patent laid-open publication Nos. 247205/87, 53401/88 and 53451/88 have disclosed methods of using proximity sensors to detect changes of magnetic fields, in judging occurrence of crushes on a body wall of a can.
Among these publications, the first-mentioned publication discloses a technique, in which a plurality of sets of laser beam emitter/receiver are provided in a row extending along and in the height direction of a can and surface irregularities of the body wall of the can are detected in terms of amounts of random reflection of laser beams. The second-mentioned publication discloses a technique, in which a plurality of sets of laser beam emitter/receiver are provided in a row extending along and in the height direction of the can and surface irregularities of the body wall of the can are detected in terms of changes of distance between each two portions diametrically opposite each other within the body wall. The third and fourth-mentioned publications disclose a common technique in sofar as surface irregularities of a body wall of a can are detected by means of a plurality of sets of light emitter/receiver provided in a row extending along and in the height direction of the can. However, the method as disclosed in the third-mentioned publication relies on changes of distance between each two portions diametrically opposite each other within the body wall while the disclosure in the forth-mentioned publication relies on the difference in the amount of light received by respective adjacent receivers. The fifth-mentioned publication discloses a technique, in which a plurality of proximity sensors are provided in a row extending along and in the height direction of a can and surface irregularities of the body wall of the can are detected in terms of electrical signals. The sixth-mentioned publication discloses a technique, in which a plurality of elongated eddy current displacement sensors are provided in a row extending along and in the height direction of a can and surface irregularities of the body wall of the can are detected in terms of amounts of displacement. The seventh-mentioned publication discloses a technique, in which a plurality of proximity sensors are provided in a row extending along and in the height direction of a can and surface irregularities of the body wall of the can are detected from the difference of electrical signals generated by adjacent sensors.
For effectively judging occurrence of crushes on a body wall of a can according to those known methods, large numbers of sets of light emitter/receiver or proximity sensors have to be provided so as to cover, in both diameter length and height direction, substantially whole portions of the body wall of the can which is liable of crushing. Thus, an apparatus employing any one of such known methods essentially becomes a complicated one.
The inventor conducted extensive research and studies in an attempt to overcome such disadvantages of the known methods and has confirmed through experiments that, in a case that an end closure is double-seamed on each of can bodies being the same in height, if a body wall of a can is crushed, the can is subjected to lower clamping force, i.e. lower pressure of a spring supporting a lifter plate, when it is being held between a chuck and the lifter plate of a double seamer.
As is well known in the art a seamer is provided with a chuck and a lifter plate aligned vertically with each other and seaming roll set near the chuck, assembling of a lid on a body wall of a can is effected by a double seaming process in which seaming rolls are forced against the flange of the lid while the body wall and the lid are being clamped between said chuck and lifter plate by means of a compression spring which supports the lifter plate. It is also well known that in order to obtain a normal double seam, the body wall has to be held against the lid with a predetermined amount of force during the double seaming process. This predetermined force is given by the aforesaid spring. A can being free from crushes on the body wall is seamed under predetermined force. If the body wall is crushed, however, the can height is reduced or the body hook of the body wall becomes shorter, so that the spring supporting the lifter plate to hold the body wall against the lid is subjected to a smaller amount of compression, resulting in smaller amount of spring force of the lifter plate to hold the body wall against the lid. For example, where the predetermined force is 100 kgf, the spring force of the lifter plate becomes 40 to 80 kgf or 40 to 80% of the predetermined force if the body wall is crushed.